Full-text retrieval has become common with coming into widespread use of the Web retrieval system of which Google (registered trademark) is an example. Depending on the using methods, it has become possible to acquire the highly fresh information by retrieving the information with the use of a retrieval system, rather than by using bookmark functions, provided in pre-existing browsers, or by using a variety of portal sites.
As regards this full-text retrieval, the scheme of preparing indexes (clues) in advance and retrieving electronic files at a high speed with the use of the so prepared indexes is now the mainstream.
Non-Patent Document 1 describes a method for preparing an inverted file which is most common as the aforementioned indexes.
The indexes, prepared by such system, are generally non-compressed and of a large capacity equal to 200 to 300% of the original document. Moreover, to keep freshness of the results of retrieval, it is necessary to dynamically update the indexes in dependence upon the state of the data being retrieved.
For example, if an index was prepared one day before, a state of data as the subject for retrieval, registered or updated after such time, is not reflected. As a matter of course, the results of retrieval are those one day before, resulting that newly registered data is overlooked or attempts are made in vain to access deleted data.
In light of above, there is disclosed in Patent Document 1 a document retrieval apparatus in which an index relevant to a newly registered document file is prepared and stored in a memory. A document retrieving section retrieves the document file using both the index data of data of the new document stored in the memory and index data of pre-existing document files stored in a disc apparatus.
There is also disclosed in Patent Document 2 a document retrieval apparatus in which, in registering a novel document in an index, such document is registered in a sub-index smaller than a main index. Each index is accessed and retrieved, in the course of retrieval, and the results are combined together to form the results of retrieval.
Patent Document 3, as another related technique, discloses a query response system, which is a sort of the information retrieval system. On the other hand, Patent Document 4, as still another related technique, discloses a chronological data processing apparatus. The query response system, disclosed in Patent Document 3, includes a relative expression analysis means and a response retrieving means. The relative expression analysis means analyzes relative expressions with respect to an axis, including the time axis, from a user's query containing the relative expressions, in order to find out an accurate response to the query. The response retrieving means retrieves the responses to the query out of extracted information data.
Further, Patent Document 4 discloses a chronological data processing apparatus formed by database units resulting from splitting per unit along the time axis. Data collected by e.g. a sensor is stored in the date-based database unit to localize the indexes, referenced in the course of retrieval, for realization of the high-speed retrieval.    [Patent Document 1]    JP Patent Kokai JP-A-9-223152    [Patent Document 2]    JP Patent Kokai JP-A-7-146880    [Patent Document 3]    JP Patent Kokai JP-A-2006-91993    [Patent Document 4]    JP Patent Kokai JP-A-7-325839    [Non-Patent Document 1]    Kenji KITA, Kazuhiko TSUDA and Masamiki SHISHIBORI, ‘Information Retrieval Algorithm’, Kyoritsu Shuppan, Jan. 1, 2002, pp. 6 and pp. 160˜179